Fouling of Reverse Osmosis and Forward Osmosis Membranes by Humic Acid – Similarities and Differences

Pressure-driven Reverse osmosis (RO) membranes have been widely used in water and wastewater treatment and seawater desalination. One major problem in RO applications is membrane fouling. It has been well documented that RO fouling is dependent on solution composition, hydrodynamic conditions, and membrane properties. Compared to RO, forward osmosis (FO) is an emerging membrane process that is driven by osmotic pressure difference (concentration difference). FO can be highly energy efficient where a natural high-concentration draw solution (e.g. seawater) is available. Limited studies are available on the fouling behavior of FO membranes.

The current study systematically investigated the fouling behavior of both RO and FO membranes under cross flow conditions. Humic acid solutions under various pH, ionic strength, and divalent ion concentrations were used as feedwater. For both RO and FO, it was found that membrane fouling was determined by interplay of electrostatic interaction and hydrodynamic drag force. Severe membrane fouling was observed at elevated membrane flux and reduced repulsive force between foulant molecules (e.g, low pH, high ionic strength, and high divalent calcium concentration). The fouling behavior of FO membrane was also strongly dependent on the membrane orientation. When the porous support layer was oriented toward the humic acid containing feedwater, severe fouling occurred. In contrast, little flux reduction was observed when the dense active layer faced the feedwater. However, the latter configuration suffered from severe internal concentration polarization. Current and future developments on FO in the Singapore Membrane Technology Center will also be briefly discussed.